Heretofore, as the substrate for wallpaper of architectural decorative materials, or for a label for containers such as glass bottles, resin bottles and the like, polyolefin films and calender-molded films that comprising, as the main ingredient thereof, a polyvinyl chloride (PVC) resin have been much used. In general, these resin films alone could hardly absorb an adhesive agent by themselves, and are therefore laminated with a water-absorbing resin layer on the adhesive side thereof (for example, Patent Document 1), or are lined with any other material.
The film laminated with such a water-absorbing layer can be stuck to an adherend with a solvent-based or water-based adhesive. However, when the film laminated with such a water-absorbing layer is removed from the adherend, it partially leaves the broken water-absorbing layer on the adherend, or it may break the material of the adherend. As a result, the removed surface could not be uniform, and when a wallpaper or a label is again stuck to the side of the adherend, then the irregularities below it would stand out on the surface thereof and the appearance of the re-stuck adherend would be thereby worsened. Owing to the problem, re-sticking is impossible.
There is another problem in that the film removed in wallpaper or label changing could not be directly recycled as a material thereof. For directly recycling the removed film, the film and the adhesive must be separated from each other, or the film and the lining material must be separated from each other. In general, however, these firmly bond to each other, and separating them is difficult.
Consequently, in case where wallpapers and labels of any type are recycled, any other materials than resin would have been mixed therein and these may be thermally decomposed to generate impurities, whereby the flowable characteristics of the resin may greatly fluctuate. As a result, there occurs a severe problem of quality, and material recycling itself is therefore difficult.
For solving these problems, there have been proposed a wallpaper and a label which have been specifically so designed that the resin material and the lining material therein can be separated from each other. For example, there have been proposed a wallpaper that comprises a PVC resin layer and, as formed thereon, a coating layer of a lining material comprising a water-soluble polymer and a binder containing a vinyl chloride-based or vinyl acetate-based synthetic resin as the main ingredients thereof, which is specifically so designed that PVC and the lining material therein could be separated from each other by dipping and stirring in hot water or in an alkaline aqueous water (for example, Patent Document 2), and a laminate that comprises a lining material and a foamed and gelled PVC substrate and, as sandwiched therebetween, a PET film, a PP film or the like (for example, Patent Document 3). However, these methods require an additional step of removing the lining material with a chemical, and therefore have some problems of recycling cost rise and recycled resin material quality degradation. Consequently, these methods are not as yet put into practical use. Still now, as a result, most removed wallpapers and labels are discarded for landfilling or by incineration, and are not recycled as resin materials. This is against the recent market demand.
As a case that can be stuck to an adherend such as wall, board or the like via an adhesive but not using a lining material, there has been proposed a readily removable multilayer resin stretched film which comprises a printable surface layer (A) containing a thermoplastic resin, an inorganic fine powder and/or an organic filler and, as laminated on the back thereof, a back layer (B) that contains a thermoplastic resin including a hydrophilic thermoplastic resin and a surface-treated inorganic fine powder, and in which the peeling strength of the back layer (B) is from 10 to 200 g/cm (Patent Document 4). In this reference, concretely described is an embodiment where a layer containing from 10 to 30% by weight of a crystalline polypropylene resin, from 2 to 10% by weight of a hydrophilic resin, from 13 to 25% by weight of a thermoplastic elastomer, and from 45 to 65% by weight of an inorganic fine powder of surface-hydrophilized particles is laminated on the substrate layer of the back layer (B). The embodiment would be surely favorable in that the film could be stuck to an adherend via an adhesive, even though a lining material is not used therein; however, when the film is removed from the adherend, the rupture propagation in the back layer (B) is unstable and therefore some irregularities would remain on the surface of the adherend to be an obstacle to re-sticking, and as the case may be, the rupture propagation in the back layer (B) would reach the surface layer. For these reasons, the film of the case is still required to be improved.
On the other hand, heretofore, there has been produced a label-integrated resin molded article by previously inserting a blank or a label in a mold, and then molding a container or the like in the mold by injection molding, blow molding, differential pressure molding, foam molding or the like (for example, see Patent Document 5 and Patent Document 6). The label of the type is referred to as an in-mold molding label, for which there are known a gravure-printed resin film, an offset-printed synthetic paper (for example, see Patent Document 7 and Patent Document 8), an flexoprinted synthetic paper, an aluminium label prepared by laminating a high-pressure-method low-density polyethylene or an ethylene/vinyl acetate copolymer on the back of an aluminium foil followed by gravure-printing the surface of the foil, etc.; and these have been put into practical use.
Recently, from the viewpoint of recycling plastic containers (material recycling), there has been a demand for readily separating and removing the label from a labeled plastic container produced by the use of an in-mold molding label, and there has been proposed a case where the label is so designed as to have a layer that enables interface peeling or interlayer peeling (for example, see Patent Document 9 and Patent Document 10).
In most those conventional in-mold molding labels, there is provided a heat-seal layer of high-density polyethylene or the like for bonding then to resin molded articles. In case where the material of the resin molded article is the same as the polyethylene resin of the label, the molded article could have a strong adhesion power; however, in case where the material of the resin molded article differs from that of the label, or that is, when the former is polypropylene, polystyrene, polyethylene terephthalate or the like, then the bonding power of the label to the molded article is extremely low, therefore causing a problem in that the label is readily removed from the molded article during transportation. For enhancing the adhesion between label and molded article, a label of which the heat-seal layer is formed of the same resin material as that of the molded article must be prepared, therefore providing a problem in that the label stock control would be complicated, and in addition, there has been pointed out a drawback in that, when the molding temperature of the molded article is low, a sufficient adhesion strength could not be attained between the molded article and the label and therefore the molding temperature for the molded article must be elevated and the productivity would be thereby lowered.
Given the situation, there has been proposed another case where, as the adhesive layer of the in-mold molding label, a conventional heat-seal layer of a low-melding-point resin for adhesion through thermal fusion bonding thereof is not used but a resin film having a porous surface with pores on the surface thereof is used, and owing to the anchoring effect of the molding resin to penetrate into the pores in the surface of the adhesive layer by pressure during molding, the adhesion between the label and the molded article is kept strong irrespective of the material of the molded article, therefore making it possible to label the molded article in a broad-range molding condition (for example, see Patent Document 11). However, the in-mold molding label of the type firmly adheres to resin molded articles (at 400 g/15 mm or more), and therefore it is difficult to readily peel and separate the label from the labeled resin molded article.
As described above, when labels could be readily removed from labeled resin containers that use in-mold molding labels applicable to various resin molded articles, not only the resin containers could be recycled but also printing may be applied on the surface of the adhesive layer as in Patent Document 10, and the print could be therefore confirmed after removal of the label, or that is, additional functions may be imparted to labeled resin containers for forgery prevention and recycling prevention of those containers, as well as secondary utilization of the removed labels for coupon tickets and others. Consequently, further improvements of labeled resin containers are desired.